Air Quality Before and After COVID-19 Lockdown Phases Around New Delhi, India

Background. The COVID-19 pandemic has had a deep global impact, not only from a social and economic perspective, but also with regard to human health and the environment. To restrict transmission of the virus, the Indian government enforced a complete nationwide lockdown except for essential services and supplies in phases from 25 March to 31 May 2020. Ambient air quality in and around New Delhi, one of the most polluted cities of world, was also impacted during this period. Objective. The aim of the present study was to assess and understand the impact of four different lockdown phases (LD1, LD2, LD3 and LD4) on five air pollutants (particulate matter (PM) PM2.5, PM10, nitrogen oxide (NOx), sulfur dioxide (SO2) and ozone (O3)) compared to before lockdown (BLD) at 13 air monitoring stations in and around New Delhi. Methods: Secondary data on five criteria pollutants for 13 monitoring stations in and around New Delhi for the period 1 March to 31 May 2020 was accessed from the Central Pollution Control Bard, New Delhi. Data were statistically analyzed across lockdown phases, meteorological variables, and prevailing air sources around the monitoring stations. Results. Pollutant concentrations decreased during LD1 compared to BLD except for O3 at all stations. PM2.5 and PM10 remained either close to or higher than the National Ambient Air Quality Standards (NAAQS) due to prevailing high-speed winds. During lockdown phases, NO2 decreased, whereas O3 consistently increased at all stations. This was a paradoxical situation as O3 is formed via photochemical reactions among NOx and volatile organic compounds. Principal component analysis (PCA) extracted two principal components (PC1 and PC2) which explained up to 80% of cumulative variance in data. PM2.5, PM10 and NO2 were associated with PC1, whereas PC2 had loadings of either O3 only or O3 and SO2 depending upon monitoring station. Conclusions. The present study found that air pollutants decreased during lockdown phases, but these decreases were specific to the site(s) and pollutant(s). The decrease in pollutant concentrations during lockdown could not be attributed completely to lockdown conditions as the planetary boundary layer increased two-fold during lockdown compared to the BLD phase. Such restrictions could be applied in the future to control air pollution but should be approached with caution. Competing Interests. The authors declare no competing financial interests.


Introduction
Economies across the world came to halt after the World Health Organization (WHO) released the news of the outbreak of a new virus, SARS-CoV-2 (coronavirus) on 5 January 2020 and later declared a global pandemic on 12 March 2020. 1,2 In India, the number of infections has crossed the 9.8 million mark with 140 000 deaths through 12 December 2020 since the first case of COVID-19 in India was reported on 30 January 2020 in the state of Kerala. 3 Like other countries, India adopted lockdown strategies to restrict the spread of the virus. Public movement and transport (road/rail/air) were restricted, and industrial operations, governmental, as well as private institutions, offices, and shopping complexes were shut. Only essential services (medical, banking, daily consumables, media, communications etc.) remained in operation during the lockdown phases. Air pollution scientists considered this unexpected and sudden lockdown to be a blessing in disguise for combating air pollution. 4,5,6,7,8, 9 Wuhan city in China observed a nearly 63% and 35% reduction in nitrogen dioxide (NO 2 ) and particulate matter (PM) concentrations in the ambient atmosphere, respectively, with limited effects on sulfur dioxide (SO 2 ) and carbon monoxide (CO) after lockdown Research began on 23 January 2020. 10 Similarly, the town of Southampton in the United Kingdom observed a 92% reduction in NO 2 emissions due to lockdown on 23 March 2020 compared to the same period from 2017-19. 10 In general, studies emerging from different countries have reflected a reduction in air pollution during lockdown periods.
In India, a complete lockdown, termed the Janta Curfew, was imposed on 22 March 2020 followed by complete lockdown phase 1 (LD1)  April 2020) with an almost complete ban on all activities except for essential supplies. In LD2 (15 April-3 May 2020), selected agricultural and industrial activities were allowed, and in LD3 (4-17 May 2020) and LD4 (18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31) May 2020), industrial and construction activities and other limited operations were allowed. These measures had a significant impact on criteria pollutants in ambient atmosphere. 5, 11 Previous reports observed a nearly 44% reduction of PM 10 , 8% of PM 2.5 , 44% of nitrogen oxide (NO X ) , and 32% of CO in New Delhi during the Janta curfew. 12 This sharp decline in pollution levels was due to restrictions on transport services, construction work and industrial activities. Air quality index (AQI) values varied between satisfactory to moderate during the lockdown phases in the National Capital Region (NCR) of New Delhi. 12 Overall, decreases of 43%, 31%, 18%, and 10% in PM 2.5 , PM 10 , NO 2, and CO levels, respectively, were reported during lockdown in India, whereas ozone (O 3 ) showed a 17% increase in concentration and SO 2 showed site-specific variations. 5 Kolkata, in eastern India, experienced a reduction of 24% to 45% in emissions of CO during lockdown due to limited industrial emissions and transport services. 13 Similarly, AQI at Silicon Valley (Bengaluru) in southern India improved from the 'hazardous' category after lockdown. 14 A nearly 40-50% reduction in NO 2 levels were reported in New Delhi and Mumbai during lockdown. 4,15 During lockdown, particulate matter concentrations were at the lowest recorded levels in the previous 20 years. 4,15 A significant reduction in air pollutants was observed during lockdown compared to 2017 and 2019 data over Kanpur, the Indo-Gangetic Plain and northern India. 16, 17 The lockdown provided an opportunity to observe and study the impacts of reduced emissions from different sources on ambient air quality. Such studies can help us to control air pollution in the future. New Delhi and its surrounding regions are among the most polluted parts of India and therefore, the present study aimed to observe the response of criteria pollutants during lockdown phases compared with the period immediately prior as well as with the same period in previous years.
In the present study, the responses of five criteria pollutants: PM 10 , PM 2.5 , SO 2 , NO x, and O 3 were observed and analyzed for the period before lockdown (BLD: 1-24 March 2020), and the different lockdown phases: LD 1 (25 March-14 April 2020), LD2 (15 April-3 May 2020), LD 3 (4-17 May 2020) and LD 4 (18-31 May 2020). A comparison of air quality data for March to May 2020 was made with air quality data during the same period in 2019.

Methods
The present study was carried out using secondary data collected from the Central Pollution Control Board (CPCB) for five criteria pollutants during the period before lockdown (BLD) and during different phases of lockdown in and around New Delhi. Meteorological data (temperature, relative humidity, planetary boundary layer and wind speed) were used to understand their possible role in variations in mass concentrations of pollutants. Furthermore, principal component analysis (PCA) was used to extract the number of principal components based on inter-pollutant associations and each component was assigned as a possible source. Principal component analysis was carried out on all five pollutants for all thirteen monitoring stations using the Statistical Package for the Social Sciences (SPSS) software version 20 using the varimax rotation method. Only components with an eigen value greater than unity were considered following the Kaiser criteria. 18 (Table 1). An important meteorological feature that changed was the PBL, which was increased by more than two-fold across all stations during the lockdown phase compared to BLD.

Research
Yadav et al  Figure 2). Similarly, PM 2.5 and PM 10 concentrations during LD1 to LD4 were lower at all stations compared to the corresponding period in the previous year (2019) (Supplemental Material Figure 2).  Figure 1). Compared to previous year observations during the same period, NO 2 concentrations were lower at all stations except for Sonipat and Panipat along the national highway (Supplemental Material Figure 2).  Figure 2).

Conclusions
Air quality improved significantly in and around New Delhi after the implementation of lockdown compared to before lockdown. However, improvements in air quality due to lockdown were monitoring station and pollutant-specific. The decrease in pollution levels could not be explained entirely based on restrictions on emission sources during lockdown. Increased wind speed and PBL were also possible reasons for dispersal of pollutants and lower concentrations in ambient atmosphere. Decreased pollution load due to lockdown may lead to improvements in human health, but caution should be taken with implementation of such lockdowns in the future, as they have negative impacts on the economic and social environment of the country.